A high resolution photoresist is required, as processing of semiconductor substrates moves toward high integration. However, it is known that a negative type resist consisting of a cyclized rubber and bisazide causes swelling during development which deteriorates resolution. In a positive type photoresist consisting of a novolak resin and an anti-resolution agent consisting of naphthoquinonediazide and other compounds, such swelling does not take place and a high resolution can be obtained. Accordingly, at present, in fine processing of a semiconductor lithography, the positive type resist is used, is primarily employed.
There is generally used as a positive type photoresist composition, a composition containing an alkali soluble resin and a naphthoquinonediazide compound as a light-sensitive substance. For example, a novolak type phenol resin/naphthoquinonediazide-substituted compound is disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470, and an example of a novolak resin consisting of cresol-formaldehyde/trihydroxybenzophenone-1,2-naphthoquinone-diazide sulfonic acid ester as a typical composition is disclosed in "Introduction to Microlithography" written by L.F. Thompson (ACS Publication, No. 2, vol. 19, pp. 112 to 121).
A novolak resin is particularly useful for a photoresist use since it can be dissolved in an alkaline aqueous solution without swelling, and when a formed image is used as a mask for etching, it shows high durability, particularly to plasma etching. A naphthoquinonediazide compound itself used for a light-sensitive material acts as an anti-dissolution agent which lowers the alkali solubility of the novolak resin. However, it has the characteristic that it is decomposed by irradiation of light to generate an alkali soluble substance, so that it raises the alkali solubility of the novolak resin. The naphthoquinonediazide compound is particularly useful as a light-sensitive material for a positive type photoresist because of this large quality change by light irradiation.
As a result, many positive type photoresists containing the novolak resin and naphthoquinonediazide series light-sensitive material have been developed and put to practical use, and sufficient results have been obtained for line width processing of about 1.5 to 2 .mu.m.
In an integrated circuit, however, integration thereof is increasing more and more, and in manufacturing a semiconductor substrate such as an ultra LSI, there is a need for processing an ultra fine pattern consisting of a line width of 1 .mu.m or less. Such an application requires a photoresist having particularly high resolution, high reproduction accuracy for a pattern form to accurately reproduce a mask for exposure, high sensitivity to improve productivity, and excellent storage stability.
It has been proposed to increase the content by weight % of a quinonediazide group in order to achieve high resolution. There is described in, for example, JP-A-60-158440 (the term "JP-A" as used herein means an unexamined published Japanese patent application), a method in which a light-sensitive substance having a high polyester content is used. Further, it has been proposed in JP-A-61-118744 to increase the number of quinonediazide light-sensitive groups contained in one molecule of a light-sensitive substance.
However, there is a problem in that these light-sensitive substances are insoluble in conventional solvents and even when they are dissolved therein, the deposits are generated during the storage of a photoresist composition to cause problems at the photofabrication and device manufacturing steps, which results in a deterioration of the yield.
JP-A-61-260239 discloses a method in which an organic solvent having a solubility parameter of 11 to 12 is added in order to prevent deposition of the light-sensitive substance. However, there is a defect that the use of a solvent such as dimethylsulfoxide and others deteriorates storage stability of the photoresist composition and sensitivity and resolution are varied to a large extent over time.
In order to achieve high resolution, it has been proposed to shift the wavelength of the light source used to a shorter range. That is, it has been investigated to use conventional g rays (436 nm) and i rays (365 nm) as well as Deep-UV rays (200 to 300 nm) and KrF excimer laser light (248 nm). In order to correspond to the above wavelengths for forming a resist pattern having good configuration, there is a need for a photoresist that has small absorption in these wavelength regions and a large bleaching ability at the exposing wavelength. However, in the case of a conventional naphthoquinonediazide light-sensitive material, the requirement for shifting the wavelength of the light source to a shorter range can not be met because of the large absorption in the Deep-UV region as well as the small bleaching performance.
Further, these quinonediazide compounds are decomposed by irradiation with actinic rays to generate five membered-cyclic carboxylic acids and become alkali soluble. However, there is a defect in that the compounds have insufficient light-sensitivity. This is because the quantum yield of quinonediazide does not exceed 1.
A method for increasing light sensitivity of a light-sensitive composition containing a quinonediazide compound has been proposed, but it was too difficult to raise light sensitivity while keeping a development latitude in a development.
Meanwhile, several proposals have been made regarding a positive light-sensitive composition which acts without using a quinonediazide compound. For example, a polymer compound having an ortho-nitrocarbinol ester group is described in, for example, JP-B-56-2696 (the term "JP-B" as used herewith means an examined Japanese patent publication). In this case, however, it does not have sufficient light sensitivity for the same reason as when using the quinonediazide compound.
Further, there is a method in which a light-sensitive system, activated by contact catalysis, is used to raise light sensitivity. This is a conventional principle in which the second reaction is caused by acid generated by photodecomposition to thereby solubilize an exposed area.
Examples thereof include the combination of a compound generating acid by photodecomposition with acetal or an O, N-acetal compound (JP-A-48-89003), the combination with ortho-ester or an amidacetal compound (JP-A-51-120714), the combination with a polymer having an acetal or ketal group on a principal chain (JP-A-53-133429), the combination with an enol ether compound (JP-A-55-12995), the combination with an N-acyliminocarbonate compound (JP-A-55-126236), the combination with a polymer having an orthoester group on a principal chain (JP-A-56-17345), the combination with a silyl ester compound (JP-A-60-10247), and the combination with a silyl ether compound (JP-A-60-37549 and JP-A-60-121446). These combinations have quantum yields theoretically exceeding 1 and therefore, they have a high light sensitivity.
Similarly, a system, which is decomposed by heating in the presence of acid to become alkali soluble while remaining stable over time at room temperature, includes, for example, the combined systems of compounds generating acids by exposing and esters having tertiary or secondary carbons (for example, t-butyl and 2-cyclohexenyl) or carbonic acid esters, described in, for example, JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, and JP-A-63-36240, Polym. Eng. Sci., vol. 23, p. 1012 (1983), ACS. Sym., vol. 242, p. 11 (1984), Semiconductor World, September 1987, p. 91, Micromolecules. vol. 21, p. 1475 (1988), and SPIE, vol. 920, p. 42 (1988). These systems also have high sensitivities and have small absorptions in the Deep-UV region as compared with a naphthoquinonediazide/novolak resin system. Accordingly, they can be an effective system for shifting the wavelength of a light source to a shorter range as described above.
Furthermore, there is a problem in that the (1) change in the lapse in time from after exposure until development and (2) the change in conditions (heating temperature and heating time) at the postexposure heating step (PEB) to promote decomposition by acid, both contribute to a variation in sensitivity and configuration of the resist pattern to a large extent.
In addition, in a pattern transfer for processing, it is desired according to the kinds of patterns or masks to be formed, that the resist forms a negative image rather than a positive image. A negative type resist containing a photolinking agent such as bisazide and other agents with a novolak resin is known, but there is a serious defect, in that the sectional shape of the resist pattern is an inverted trapezoid because of the narrow development latitude and the large ray absorption by the photolinking agent.
Further, there is a method in which a photo acid generating agent is added to a novolak resin in combination with an additive causing with an acid as a catalyst a curing reaction to a binder such as a novolak resin and other binders, to obtain a negative type light-sensitive resin composition. In this method, an increase in yield in the chemical reaction based on a catalytic reaction can be expected at the reaction process following exposure, and therefore the amount of a light-sensitive agent added, that is, the amount of the light absorption component, can be decreased. Accordingly, it is possible to solve the above problem of the sectional shape characteristic of the negative type image, and it is promising as a resist material for fine processing. Further, high quantum yield, that is, high sensitivity, can be expected because the reaction system is accompanied by an increase in chemical reaction yield. Accordingly, if this light-sensitive composition is used as a light-sensitive layer for lithography, it can be applied to a high sensitive printing plate suitable for exposing with a laser ray.
The following references are known for such a photocurable resin composition.
JP-B-54-23574 discloses a technique for photocuring a novolak resin in combination with a photo acid generating agent comprising an organic halogen compound.
German Patent Publication 2057473 discloses a photo acid generating agent consisting of a diazo compound and a phenol resin such as novolak as a binder for a photocurable composition consisting of methylolized melamine and others.
JP-A-60-263143 discloses a composition comprising an acid curable amino blast resin consisting of a photo acid generating agent, a melamine resin and others, and a conventional novolak resin, and describes a negative image, which is capable of aqueous development and has high heat stability.
JP-A-62-164045 discloses that an organic halogen compound having photoabsorption in the far UV region can be used as a photo acid generating agent for such a composition. Similarly, JP-A-2-52348 discloses that an organic halogen compound having a pKa value falling within a specific range is advantageous as a photo acid generating agent of a similar system.
JP-A-2-154266 discloses that oximesulfonic acid esters are effective as a photo acid generating agent for a similar photocurable composition. Further, JP-A-2-146044 discloses a composition prepared by combining a photo acid generating agent having a specific trichlorotriazine group with a novolak resin containing m-cresol of 30% or more in alkoxymelamine is advantageous for exposure with a high energy ray. Further, European Patent 397460A discloses that a novolak resin having a high degree of branching is used in a similar composition.
These publications do not contain much description regarding an acid curable light-sensitive resin composition in which the same novolak resin is used while attempting to obtain high resolution by applying it to a short wavelength. This is partly due to the fact that a lot of these publications can not be utilized as they are since a guiding principle for designing a function of a molecule and a material system for obtaining a high resolution in a negative resist is fundamentally different from that of a positive type.
It is disclosed in, for example, JP-A-2-146044 that in an acid curable type negative resist in which a novolak resin is used, one in which a novolak resin having a high m-cresol content is used gives a high residual layer rate and good pattern configuration, but this does not necessarily apply to a positive resist. Accordingly, a material must be searched by trial and error for attempting a higher resolution of an acid curable type negative resist, and it still remains difficult to precisely make a technical estimate on the relationship between the material applied and the resolution.
Further, there is a problem in the above negative type resist in which a photo acid generating agent and an acid curing agent are combined. Specifically, when the lapse of time from exposure until development is changed due to a diffusion of acid generated by irradiation of light or when the conditions (a heating temperature and a heating time) at a post-exposure baking process (PEB) are changed in order to promote acid curing, sensitivity and configuration of the resist pattern are varied to a large extent.